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Rainfall thresholds for streamflow generation are commonly mentioned in the literature, but studies rarely include methods for quantifying and comparing thresholds. This paper quantifies thresholds in ephemeral streams and evaluates how they are affected by rainfall and watershed properties. The study sites are in southern Arizona, USA; one is hyperarid and the other is semiarid. At both sites rainfall and streamflow were monitored in watersheds ranging from 10(-3) to 10(2)km(2). Streams flowed an average of 0-5 times per year in hyperarid watersheds and 3-11 times per year in semiarid watersheds. Although hyperarid sites had fewer flow events, their flow frequency (fraction of rain events causing flow) was higher than in semiarid sites for small (<1km(2)) watersheds. At both locations flow frequency decreased with drainage area, but the decrease was steeper in hyperarid watersheds. Watershed mean 60-min intensity thresholds ranged from 3-13mm/hr in hyperarid watersheds and 7-16mm/hr in semiarid watersheds. Higher runoff thresholds and lower flow frequencies in small semiarid watersheds likely relate to greater ground cover and soil development compared to the desert pavement and bedrock surfaces in hyperarid sites. The choice of rain data strongly influenced threshold values; single rain gauges were only adequate for threshold prediction in <1-km(2) watersheds, and incomplete rainfall data led to increases in thresholds with drainage area. We recommend using mean rainfall intensity over the drainage area for threshold analysis because this reduces apparent scale dependence in thresholds caused by incomplete rainfall information.

Plain Language Summary Ephemeral streams in deserts are usually dry, flowing only after heavy rains. Our goal was to determine how much rain is needed for these streams to flow. We studied streams in dry southwestern and in wetter southeastern Arizona, USA. The dry site has mostly bare rock across the landscape, whereas the wetter site has more vegetation, including shrubs, grasses, and oak. In the dry area, small streams flowed 3-5 times per year, and larger streams flowed 0-2 times per year. Streams required 3-13mm of rain over 1hr to trigger flow. The small streams flowed more frequently because rain falling on bare rock could rapidly reach the stream. Water was lost into the channel bed before reaching larger streams. In the wetter area, both small and large streams flowed 3-11 times per year. These streams required 7-16mm of rain over 1hr to trigger flow. This range of rainfall is slightly higher than for streams in the dry study area, likely because vegetation cover and soil development allow more rainfall to infiltrate into soil before reaching streams. Information on the amount of rainfall needed to trigger streamflow can help with issuing flash flood warnings for ephemeral streams.